Cryogenic process for separation of a natural gas with a high nitrogen content



Oct. 6, J PARNAG ETAL CRYOGENIG PROCESS FOR SEPARATION OF A NATURAL GASWITH A HIGH NITROGEN CONTENT Original Flled Oct. 23. 1965 United StatesPatent Office 3,531,943 Patented Oct. 6, 1970 3,531,943 CRYOGENICPROCESS FOR SEPARATION OF A NATURAL GAS WITH A HIGH NITROGEN CONTENTJohn Parnag, Covina, John W. Menzies, San Gabriel, Frederick W.Kirkpatrick, Jr., Covina, Robert B. Ritter, San Gabriel, and John L.Sullwold, La Canada, Calif., assignors to Aerojet-General Corporation,El Monte, Calif., a corporation of Ohio Continuation of application Ser.No. 503,382, Oct. 23, 1965. This application May 22, 1968, Ser. No.732,010 Int. Cl. F25j 3/02, 3/08 U.S. Cl. 62-28 10 Claims ABSTRACT OFTHE DISCLOSURE This patent describes a process for the reduction of theamount of inerts in a mixture comprising a combustible hydrocarbon andinert gases including nitrogen which comprises: cooling a stream of highpressure gas containing combustible hydrocarbons and inert gasesincluding nitrogen; expanding the cooled stream under substantiallyisentropic conditions to cool the gas and to remove work therefrom;passing the expanded stream into a separation zone operated atsuiciently high pres-- sure to permit the use of methane at its boilingpoint to remove heat from the top of said zone; forming said methane atits boiling point by the steps comprising cooling and condensing propanewith water, condensing and sub-cooling propane with cold product streamsand recycled low pressure propane refrigerant vapor, cooling andcondensing ethylene with water, cold product streams, and vaporizingpropane, sub-cooling ethylene with cold product streams and recycled lowpressure ethylene refrigerant vapor, cooling methane with Water, coldproduct streams, and recycled 'low pressure methane refrigerant vapor,condensing methane with evaporating ethylene and evaporating hydrocarbonproduct and sub-cooling methane with cold product streams and recycledlow pressure methane refrigerant vapor; passing said methane to the topof said zone, and removing substantially pure nitrogen from the top ofsaid zone and substantially pure methane from the bottom of said zone.

This application is a continuation of application S.N. 503,382 led Oct.23, 1965 and now abandoned.

This invention pertains to the removal of inert gases such as nitrogenfrom gas streams containing combustible hydrocarbons. More particularlythe invention is concerned with the removal of inert gases from naturalgas to increase its heating value and provide for the recovery of thevaluable inert gases.

Hydrocarbon rich gases such as natural gas are widely used for heatingpurposes. In general, in this country, a commercially acceptablematerial must have a heating value of about 1000 B.t.u. per standardcubic foot. However, there exists in many places, particularly in theGreat Basin area of the United States, vast reserves ofhydrocarbon-containing gases which are diluted with substantial amountsof inert gases such as nitrogen, carbon dioxide, helium, argon, and thelike. The presence `of large amounts of inert gases in these materialsrenders them practically useless as fuel because of their low heatingvalue. Accordingly, various attempts have been made to remove the inertgas from these gas reserves to increase the heating value.

Certain of the inert gases present in the low utilityhydrocarbon-containing gases may be readily removed. For example, carbondioxide can be effectively eliminated from the gas by treatment with hotpotassium carbonate. Similar procedures are effective for the removal ofhydrogen sulfide. However, these techniques are ineffectual in removingnitrogen which is often the principal inert material in the low utilitygases. The nitrogen, as a practical matter, must be removed by acombination of refrigerative and distillative processes. Generally,these separation techniques involve the separation of nitrogen (normalboiling point -320.5 F.) from methane (normal boiling point -258.7 F.),the principal hydrocarbon in the 10W utility gases. The separation ofthese gases on large scale in an economical fashion has been a longstanding problem in the art, restricting the commercial exploitation andutilization off the existing gas reserves containing inerts. Theeconomics of the separation is a function of the total amount of workrequired to compress and cool the gases to a liquid state whereseparation by distillation is possible. In the typical process of theprior art as shown in U.S. Pat. No. 2,583,090, the cooled high pressurefeed gas is throttled into a rectification tower where essentially puremethane is recovered at low pressure. This involves an isenthalpic orJoule-Thomson expansion. However, this technique does not produce anywork from the expansion step. Moreover, the process employing theisenthalpic expansion is characterized by other inefficiencies, sincefor example, it is not possible to recover valuable liquid nitrogen fromthe tower without the benefit of costly lower level refrigeration in theprocess.

It has also been proposed to isentropically expand the high pressurefeed gas before cooling and throttling into a rectification column. Wehave found that this procedure results in less eiiicient heat transferin cooling of the gas yand also requires costly low level refrigerationin order to recover liquid nitrogen from the rectification column.

According to this invention, it has been found that substantiallyincreased efliciencyis obtained by cooling the high pressure feed gas,isentropically expanding into a separation column operated at a pressuresuciently high to condense the separated nitrogen by heat transfer withboiling methane at a pressure at least as high as atmospheric, andrecovering essentially pure nitrogen and methane. In this way, goodseparations are achieved and both the liquid nitrogen and methane areobtained with the least amount of refrigeration and other compressionwork.

Accordingly, it is a principal object of this invention to provide aprocess for the removal of inert gases from hydrocarbon-containing gasesto provide a saleable high heating value gas in a more efficient andeconomical manner.

More particularly, it is an object of this invention to provide for theseparation of hydrocarbons from inerts in low utility gases employing anisentropic expansion step, and boiling hydrocarbon at a pressure of atleast one atmosphere as the refrigerant in the separation zone.

Still another object of the present invention is to provide a puricationprocess for low utility gases which yields an acceptable fuel gas havinga heating value of about 1000 B.t.u. per cubic foot, and additionallyprovides for the commercially feasible recovery of the separatedessentially pure nitrogen in liquid or compressed form.

It is also an object of our invention to separate methane Aand nitrogenin a more eicient manner.

In another aspect, it is an object of the invention to separate andrecover hydrocarbons and inert material in a process utilizing a novelcascade refrigeration system.

from natural gas, as well as the recovery of hydrogen from manufacturedor synthetic gases.

These and other objects of the invention will be apparent from thedetailed description which follows, taken in conjunction with theaccompanying drawing.

Briefly, the present invention comprises a process for the reduction ofthe amount of inerts in a mixture comprising a combustible hydrocarbonand inert gases including nitrogen, which comprises cooling a stream ofhigh pressure gas containing combustible hydrocarbons and inert gasesincluding nitrogen, expanding the cooled stream under substantiallyisentropic conditions to remove work therefrom, passing the expandedstream into a separation zone operated at sufliciently high pressure topermit the use of methane at its boiling point under a pressure,preferably at least as high as one atmosphere, to remove heat from thetop of said zone, separating said expanded stream in said column intosub pure nitrogen and sub pure methane, and removing liquid and/orgaseous substantially pure nitrogen from the top of said zone and liquidand/ or gaseous substantially pure methane from the bottom of said zone.The invention includes the further steps of providing at least part ofthe methane for removal of heat from the top of the separation zone bymethane liquefaction in a novel cascade refrigeration system utilizingcold product streams. In another aspect, our invention comprises therecovery of high purity argon and/or helium from the nitrogen removedfrom the top of the separation zone. In yet another aspect, theinvention comprises the recovery of hydrogen from said expanded andseparated stream.

The process of the present invention can be better understood by theaccompanying drawing depicting an illustrative embodiment of theinvention.

As shown on the drawing, any hydrocarbon-bearing gas containing inertsand non-hydrocarbons such as nitrogen, carbon dioxide, sulfur compoundsand helium is gathered from gas wells and transferred to the plant. Thegas is first treated to remove carbon dioxide and hydrogen sulfide byany conventional means such as monoethanolamine scrubbing. The treatedgas at point 100 has any liquid water removed in a conventionalentrainment separator 101, and is then dehydrated by conventionaldessicants in vessel 102, connected by line 103 to vessel 101 to a dewpoint of about 100 F. or lower. The gas at this point is at ambienttemperature and is passed via line 104 to heat exchanger 105 wherein thegas gives up some heat in separate passes to a counter iowing coldmethane-rich product. The cold dried gas stream is then passed fromexchanger 105 into line 106,

boiler for distillation tower 109. Heat gained by the methane andheavier hydrocarbons in the bottom of tower 109 causes vaporization ofthe liquids and helps in the distillation of the natural gas. The coldnatural gas feed from exchanger 108 flows through line 110 throughisentropic expansion engine 111, with any additional condensate beingiirst removed via separator 112. Alternatively, when none of thecondensed liquids are withdrawn via seperator 112 or via separator 107,it is necessary that all liquids in the feed be by-passed around theexpansion engine 111 via line 113. Separation of the liquid from the gastakes place in separator 11311. This is to prevent damage to expander111 by the introduction of excess liquid particles. The feed gas thendischarges from expander 111 at reduced pressure with work having beenremoved. The discharge from expander 111 contains a substantial portionof liquid on the order of up to about mole percent. The vapor-liquidmixture flows then through line 114 into the feed partial condenser 115where further liquefaction occurs as the result of a further lowering ofthe temperature. This cooling increases the amount of liquid up to about30 to 40 mole percent. This feed mixture then flows into distillationtower 109 via line 116 for separation into a nitrogen rich overheadproduct and a hydrocarbon rich bottoms product. v.In conventional mannerby proper manipulation of reboiler heat into the bottom of the tower andheat extraction from the top of the tower, as well as employment of anappropriate number of distillation stages, any desired degree ofseparation is achieved.

A portion of the bottoms liquid methane product is diverted into line117 and throttled through `valve 118 in the extension of line 117 intothe evaporator side of partial feed condenser 115. The vapor-liquidhydrocarbon mixture formed on throttling to a reduced pressure providesrefrigeration to the natural gas feed expander efuent. The totallyvaporized methane next flows through line 119 to a refrigerant methanesubcooler 120 to provide refrigeration to the liquid methanerefrigerant. In like manner and sequentially, the low pressure methaneproduct flows from exchanger 120 through line 121, methane chiller 122,line 123, ethylene chiller 124 and line 125 giving up refrigeration torefrigerant fluids in heat exchange and itself becoming warmer. Beyondline 125 the flow is divided, part continuing in line 126- to ethylenecooler 127 and the remainder continuing via line 128 through propanesubcooler 129. The split streams in lines 130 and 131 at ambienttemperature and normally about 5 F. colder than the incoming warmstreams in heat exchange, combine and leave the system in line 132. Theproduct from line 132 may be utilized for fuel for gas engines drivingcompressors as well as for the production, for example, of hydrogen foran ammonia plant.

Turning to line 133 at the bottom of tower 109, that portion of theliquid methane not diverted to line 117 ows by way of line 134 to theevaporator side of refrigerant methane condenser 135. Throttling occursacross valve 136 in line 134, the methane product is reduced in pressurein passing from line 133 to the evaporator side of unit as Well asundergoing a reduction in temperature. Evaporation of the methaneprovides a portion of the condensation duty of the methane refrigerantas hereinafter explained. Emerging as saturated vapor from unit 135, themethane at reduced pressure ows via line 137 to methane refrigerantchiller 122 and gains heat from cooling methane refrigerant.Sequentially, the methane product flows from unit 122 through line 138and natural gas feed chiller 105. Finally it emerges from the system inline 139 at ambient temperature within about 5 F. of the feed gastemperature. This methane product is normally at an elevated pressureand may be utilized as reformer feed in the production of hydrogen orammonia. The product may also be utilized for other purposes such as gaspipeline feed.

The overhead products from tower 109 are liquid and vapor nitrogen. Inthe case where liquid nitrogen product is desired, the liquid nitrogenis withdrawn from tower 109 via line 140 and subcooler 141. It is thenthrottled across valve 142 in line 143 into the nitrogen ash drum 144. Aportion remains as liquid and is subsequently taken to storage via line145. The nitrogen flash vapor from drum 144 via line 145a is used tosu'bcool liquid nitrogen in exchanger 141 prior to throttling into ashdrum 144. This low pressure vapor nitrogen is then warmed to ambienttemperature which is about 5 F. cooler than the warm high pressureethylene refrigerant entering exchanger 127 by passage through line 146,methane subcooler 120, line 147, methane chiller 122, line 148, ethylenechiller 124, and line 149, where it enters ethylene cooler 127. Thegaseous nitrogen at about one atmosphere is then ejected from the plantto the atmosphere via line 150. Some portion of the Waste nitrogen maybe conveniently used as a purge gas for adsorbent regeneration or as aninert gas for keeping insulation around the cold equipment dry. Thisnitrogen can also be compressed into commercial gas for sale.

High pressure gaseous nitrogen may also be produced simultaneously withthe liquid nitrogen. A portion of vapor nitrogen is withdrawn from thehead of the tower 109 via line 151 and is then passed through methanesubcooler 120, line 152 and methane chiller 122, providing in parallelwith other cold gaseous streams previously described, refrigerationneeded to cool methane refrigerant. Beyond exchanger 122, line 153divides into two parts, lines 154 and 15S. Part of the nitrogen isdiverted via line 155 to the ethylene refrigeration circuit. Here it owsthrough ethylene chiller 124, line 156 and ethylene cooler 127 ending asa warm gas in line 157. The second portion of nitrogen flows throughline 154 and methane cooler 158 ending as warm gas in line 159. Thesplit streams of high pressure nitrogen are both warmed to a point about5 F. cooler than the warm refrigerant streams being cooled. The twonitrogen flows are reunited in line 160 whence the single flow leavesthe systern. This high pressure gas may conveniently be used in ammoniasynthesis in unit 160:1, or further compressed into bottles for sale.

To keep the method of this invention in heat balance, an additionalamount of refrigeration exclusive of ,expander refrigeration andauto-refrigeration by-product streams, must be provided by externalmeans. This additional refrigeration is provided by a unique form ofcascade refrigeration. Starting with the lowest temperature or methaneleg of the cascade, the sequence of steps involves rst compressingmethane in a multistage compressor 161. With the heat of compressionbeing removed in intercoolers and aftercooler 162, the gaseous methanethen ows via line 163 to methane cooler 158 where it is cooled by heatexchange with cold product nitrogen and cold recycled methanerefrigerant. The methane refrigerant is further transferred from unit158 via line 164 to methane chiller 122 and there cooled to a stateproviding a saturated vapor. Recycling methane refrigerant plus severalcold product streams provide cooling at this stage. Saturated methanevapor leaves heat exchanger 122 by line 165 and then divides into branchlines 166 and 167 to the methane condensers 135 and 168. The split ofrefrigerant methane between units 135 and 136 will depend upon theamount of product liquid methane available at this point for condensingrefrigerant in unit 135. In general, the amount of heat taken up byevaporation of product is inadequate to totally condense methanerefrigerant. For this reason the remainder of the condensate load inunit 168 is carried by the ethylene loop of the cascade refrigerantsystem. The saturated liquid methane refrigerant from condensers 168 and135 flows from the respective units by lines 169 and 170 which joinsinto line 171. Line 171 conveys the liquid to methane subcooler 120where the fluid is subcooled again by itself in a cold low pressuregaseous state as well as by various cold product streams. The subcoolinglowers the enthalpy of the unthrottled refrigerant to maximize theamount of liquid in the condenser 172 after Joule-Thomson expansion.

The subcooled methane refrigerant flows from subcooler 120 by line 173to condenser 172 in the top of tower 109. Just prior to entry into theevaporative side of the condenser, throttling occurs across valve 174 inline 173. The liquid formed by throttling through valve 174 isconstantly vaporized in the condenser by heat exchange with risingvapors in tower 109, which in turn condense and provide both liquidnitrogen tower overhead product and internal reflux to effect separationby distillation. The combined lash vapors and evaporated liquid methaneleave the condenser via line 175 and ow to the suction of the methanecompressor 161 imparting refrigeration to itself in the high pressurestate sequentially through methane subcooler 120, line 177, methanechiller 122, line 178, methane cooler 158 and finally via line 179 tothe compressor 161.

CII

As auto-refrigeration and cold product streams together do not provideadequate refrigeration to liquefy and subcool methane refrigerant,additional outside refrigeration is provided by the propane and ethyleneloops of the cascade. Starting at ethylene compressor 180, ethylene gasis compressed and inter and aftercooled, discharging into line 181. Theethylene is next cooled to saturated vapor in ethylene cooler 127against cold product streams. From cooler 127, the ethylene vapor flowsvia line 182 to ethylene condenser 183. Total condensation of ethyleneoccurs against vaporizing propane. Saturated liquid ethylene fromcondenser 183 ows through line 184 to ethylene chiller 124 in whichsubcooling occurs by means of heat exchange with itself as a recycledcold low pressure gas, plus cold product streams. From chiller 124, thesubcooled uid goes to a second subcooler 185 connected to chiller 124 byline 186. This second stage of subcooling is accomplished byself-refrigeration of the high pressure liquid ethylene by itself asexpanded returning material in the cold low pressure state. Thesubcooled ethylene travels in line 187 from subcooler 185 to ethylenecondenser 168. Before entering condenser 168, it is expandedisenthalpically across valve 188 in line 187. Acquiring heat fromcondensing methane, the ethylene is vaporized. This cold low pressureethylene is returned to compressor 180 giving up refrigeration and beingwarmed by passage through line 189, ethylene subcooler 185, line 190,chiller 124, and iinally line 191 to the suction of the compressor 180.

As the ethylene leg of the cascade refrigeration system is required tocondense methane, similarly the propane leg or loop is necessary tocondense the ethylene. Propane itself is condensed by heat exchange withwater. As shown, propane is compressed in propane compressor 192 andessentially completely condensed in the aftercooler-condenser 193,preceeding line 194. Line 194 conveys the virtually liquid propane topropane subcooler 129. Residual vapor condensation and subcooling oftotal liquid propane takes place in unit 129. Heat removal from propaneis brought about by both auto-refrigeration and cold product. Thesubcooled propane from subcooler 129 in line 195 flashes through valve196 1n the evaporator side of ethylene condenser 183. The evaporatedcold low pressure vapor propane flows Via line 197, subcooler 129, andline 198 back to compressor 192 suction.

The following example is presented solely to illustrate the inventionand hence should not be regarded as limiting in any way. In the example,the parts, percentages and ratios are by weight, and pressures areabsolute, unless otherwise indicated.

EXAMPLE Natural gas containing about 60% by volume nitrogen and about39% by Volume of hydrocarbons, mostly methane, together with smallamounts of carbon dioxide, helium, argon, and hydrogen sulfide is fed ata pressure of 2000 p.s.i. to a purification treatment utilizingmonoethanolamine to remove carbon dioxide and hydrogen sulfide. Thetreated gas at point 100 in the accompanying drawing is then dried bypassage through entrainment separator 101 and further dried in vessel102 to a dew point of 100 F. The gas at this point is at a temperatureof about 90 F. At this point the gas composition is about 60.61% byvolume nitrogen, 38.86% by volume methane, 0.29% by volume ethane, 0.08%by volume propane, 0.04% by volume butane, and 0.12% by volume pentane.The dehydrated treated gas from dryer 102 flows through line 104 to heatexchanger 105 where the gas gives yup some heat to the counter-flowingmethane product. The temperature of the feed following erchanger isabout +49 F. Any condensed hydrocarbons flowing from exchanger 105 intoline 106 are removed from the system in separator 107. The chilled feedgas flows from exchanger 105 through line 106 to a heat exchanger 108(the reboiler for tower 109) and is therein further cooled to about 96F. Heat gained by the methane and heavier hydrocarbon liquids in thebottom of tower 109 causes vaporization of the liquids and aids in thedistillation of the natural gas. The cold natural gas flows fromexchanger 108 through line 110 to expansion engine 111, with any liquidsbeing separated and Withdrawn via separator 112. The natural gasdischarges from expander 111 at a pressure of 400 p.s.i., a temperatureof 197 F. and containing 15 mole percent liquids. The vapor-liquidmixture ows through line 114 into the feed partial condenser 115 wherefurther liquefaction occurs, the temperature dropping to about 203 F.and the amount of liquid increasing to about 36 mole percent. This feedmixture flows into tower 109 via line 116 for separation into a nitrogenrich overhead product and a hydrocarbon rich bottoms product. Thebottoms liquid hydrocarbon product from tower 109 is removed via line133 and contains only 0.3 by volume nitrogen. A portion of the bottomsliquid methane is diverted to line 117 and throttled through valve 118into the evaporator side of feed partial condenser 115. The vapor-liquidhydrocarbon mixture formed on throttling to about 55 p.s.i. providesrefrigeration to the natural gas feed expander effluent. The totallyvaporized methane then flows through line 119 to refrigerant methanesubcooler 120 through line 121, methane chiller 122, line 123, ethylenechiller 124 and line 125 giving up refrigeration to refrigerant fluidsin heat exchange and iitself becoming warmer. From line 125 the flow isdivided, part continuing in line 126 through ethylene cooler 127 and theremainder continuing via line 128 through propane subcooler 129. Thesplit streams in lines 130 and 131, as a result of heat exchange inexchangers 129 and 127, respectively, emerge at a temperature about F.colder than the incoming warm streams, combine, and leave the system inline 132. At line 133 at the bottom of tower 109, that portion of theliquid methane not diverted to line 117 flows by way of line 134 to theevaporator side of refrigerant methane condenser 135. Throttling occursacross valve 136 in line 134, the methane product dropping from line 133pressure of about 400 p.s.i. to about 255 p.s.i. in the evaporator sideof unit 135, and from about 141 F. to 167 F. Evaporation of the methaneprovides a portion of the condensation duty of the methane refrigerant.Emerging as saturated vapor from unit 135, the 255 p.s.i. methane flowsvia line 137 to methane refrigerant chiller 122 and gains heat fromcooling methane refrigerant. Sequentially, the methane product flowsfrom unit 122 through line 138 and natural gas feed chiller 105. Finallyit emerges from the system in line 139 at ambient temperature withinabout 5 'F, of the feed gas temperature. The overhead products fromtower 109 are liquid and vapor nitrogen having a purity of 99.9997% byvolume nitrogen, the remaining three parts per million being principallymethane. Liquid nitrogen at about 254 F. and 397 p.s.i. is withdrawnfrom the top of tower 109 via line 140 and subcooler 141. It is thenthrottled across valve 142 in line 143 into the nitrogen flash drum 144at about 17.2 p.s.i. A portion remains as liquid and is subsequentlytaken to a storage vessel by line 145. The nitrogen flash vapor fromdrum 144 via line 1-45a is used to subcool tower draw-off liquidnitrogen in exchanger 141 prior to throttling into flash drum 144. Thelow pressure vapor nitrogen routed through line 146, methane subcooler120, line 174, methane chiller 122, line 148, ethylene chiller 124, line149 and ethylene cooler 127 is warmed to ambient temperature about 5 F.colder than the warm high pressure ethylene refrigerant enteringexchanger 127. At this point, line 150, the gaseous nitrogen at oneatmosphere pressure is ejected to the atmosphere. A portion of highpressure vapor nitrogen is withdrawn from the head of tower 109 via line151 and passed in order through methane subcooler 120, line 152, andmethane chiller 122, providing in parallel with other cold gaseousstreams refrigeration needed to cool methane refrigerant. Beyondexchanger 122, line 153 divides into two parts, lines 154 and 155. Partof the nitrogen is diverted via line 155 to the ethylene chiller 124,line 156, and ethylene cooler 127. It leaves cooler 127 as warm gas line157. The second portion of nitrogen ows through line 154 and methanecooler 158 from which it leaves as warm gas in line 159. The splitstreams of high pressure (about 395 p.s.i.) nitrogen are both warmed toa point about 5 F. colder than the warm refrigerant streams beingcooled. The two nitrogen ows are reunited in line whence the single flowleaves the system. In the cascade refrigeration system, the methane isiirst compressed in multistage compressor 161 to develop a dischargepressure of about 285 p.s.i. The heat of compression is removed by waterin aftercooler 162 preceding line 163. The gaseous methane now at about+90 F. flows via line 163 to methane cooler 158 and is there cooled toabout 125 F. by heat exchange with cold product nitrogen and coldrecycled methane refrigerant. The methane refrigerant is furthertransferred from unit 158 via line 164 to methane chiller 122 and therecooled to about 162.5" F. at 282 p.s.i. (saturated vapor). The saturatedmethane vapor leaves heat exchanger 122 by line 165 and then dividesinto branch lines 166 and 167 to the methane condensers 135 and 168.Saturated liquid methane refrigerant at about 162.5. F. from condensers135 and 168 ows from the respective units by lines 169 and 170 whichjoin into line 171. Line 171 conveys the liquid to methane subcooler 120wherein the luid is subcoled to about 241.8 F. 'Ihe subcooled methanerefrigerant flows from subcooler 120 by line 173 to condenser 172 in thetop of tower 109. Just prior to entry into the evaporative side of thecondenser, throttling occurs across valve 174 in line 173. Condenserconditions are about 254.7" F. and 17.6 p.s.i. The liquid formed fromthrottling across valve 174 is constantly vaporized in the condenser byheat exchange with rising vapors in the tower which in turn condense andprovide both liquid nitrogen overhead product and internal reflux. Thecombined ash vapors and evaporated liquid methane leave the condenservia line 175 and flow to compressor 161 imparting refrigeration toitself in the hot high pressure state sequentially through methanesubcooler 120, line 177, methane chiller 122, line 178, methane cooler158 and nally via line 179 to compressor 161. In the ethylene loop,ethylene gas at 50 F. and about 7.6 p.s.i. is compressed by ethylenecompressor 180, aftercooled and discharging at +90 F. and 197 p.s.i.into line 181. The ethylene is next cooled to 45.5 F. with saturatedvapor in ethylene cooler 127 against cold product streams. From cooler127 the vapor ethylene ows via line 182 to ethylene condenser 183. Totalcondensation of etheylene occurs against vaporizing propane. Saturatedliquid ethylene from condenser 183 flows through line 184 to ethylenechiller 124 in which subcooling occurs by means of heat exchange withitself plus cold product streams. From chiller 124, the subcooled fluidgoes via line 186 to a second subcooler 185. This second stage ofsubcooling is accomplished by self-refrigeration of the high pressureliquid ethylene lby heat exchange with itself as expanded returningmaterial in the cold low pressure vapor state. The subcooled ethylenetravels in line 187 from subcooler 185 to methane condenser 168. Beforeentering condenser 168, it is expanded isenthalpically across valve 188in line 187. Acquiring heat from condensing methane, the ethylene isvaporized. This cold low pressure ethylene at 9.6 p.s.i. is returned tocompressor giving up refrigeration and being warmed to about 50 F. whilepassing through line 189, ethylene subcooler 185, line 190, ethylenechiller 124, and line 191 to the suction of the com-pressor 180. Propaneis compressed in propane compressor 192 and essentially completelycondensed iby heat exchange with water in aftercooler 193 preceedingline 194 to a temperature of |90 F. and a pressure of 170 p.s.. By line194 the propane, which is virtually 100% liquid, is conveyed to propanesubcooler 129. Residual vapor condensation and subcooling of totalliquid propane takes place in unit 129. Heat removal from propane isbrought about by both auto-refrigeration and cold products. Thesubcooled propane from unit 129 and line 195 flashes through valve 196into the evaporative side of ethylene condenser 183. The evaporated coldlow pressure vapor propane from unit 183 ows via line 197, subcooler 129and line 198 back to compressor 192 suction.

While the foregoing discussion has been with particular reference to theupgrading of natural gases, the present invention is in no Way limitedto natural gas. There exist many synthetic gas mixtures formed asprimary raw material sources or by-products of various coking, cracking,reforming and partial oxidation processes. The raw gas mixtures must befurther refined to retain desired valuable constituents. The describedinvention with little or no modification lends itself quite readily toseparating such synthetic gas mixtures. The present invention possessesthe flexibility to efficiently handle mixtures of varying composition aswell as of varying volume. It is also possible to combine the eluentfrom several adjoining reneries and/or chemical plants, with thecombined eflluent being piped to a single gas separation unit having thedesign of the present invention.

As indicated above, the location of the expansion engine 111 in thisinvention, that is, after the cooling of the gas -but prior to injectioninto the column 109, forms an essential feature of the process. If theexpansion engine 111 were located at or near 100, the removal of work bythe expansion engine would be increased, however, the subsequent removalof heat from the incoming gas would be less et'rcient because the gaswould be at a lower pressure, and hence, the heat transfer coelicientswould be less favorable.

The use of expansion engine 111 to reduce the pressure of the cold gaspossesses many other advantages unattainable by the use of isenthalpicthrottling or by the use of an expansion engine located at some otherpoint in the process. According to the process of the present invention,the cold feed is isentropically reduced in pressure across expansionmachine 111. It will be understood that while this expansion is ideallyisentropic, as a practical matter, the isentropic efficiency of expander111 is usually on the order of about 70%. The advantages obtained byisentropic expansion of cold feed gas will be -better understood uponanalysis of the thermodynamic considerations associated with expansionunder various conditions. For example, for a 60 mole percent nitrogen 40mole percent methane mixture, if the feed temperature to the expander111 is at about 25 F. and 2000 p.s.., upon expansion to 400 p.s.. thefinal gas temperature would be relatively high (about 165 F.) with 710.5B.t.u./lb. mole of gas being obtained as Work (assuming expandereiciency of 70%). `On the other hand, when the gas is precooled to 96 F.at 2000 p.s.., and then isentropically expanded at 70% efficiency to400p.s.i., the discharge temperature is low (about 197 F.) and the workproduced is 384.9 B.t.u./lh. mole of gas.

If the precooled feed gas were throttled into a distillation tower byisenthalpic expansion, no work would be produced by such expansion.However, by the practice of this invention work isproduced at the shaftof expander 111 for use in driving other machinery in the operation,such as, refrigerant and product compressors. Isenthalpic expansion hasother disadvantages since the least level of refrigeration attained uponexpansion of the gas is about 198 F. including cooling of the feed inexchanger 115. On the other hand, the use of expander 111 gives a gasdischarge temperature (for a 60:40 nitrogen-methane mixture) of about197 F. which followed by cooling in exchanger 115 reduces the gastemperature to about 203 F. This is due to the fact that Whileisenthalpic expansion can sometimes approach the cooling effect obtainedby isentropic expansion, it can never reach this cooling level. It willbe understood that the foregoing discussion is with reference to amixture containing 60% nitrogen and 40% methane. Other proportions ofthe nitrogen and methane necessitate other temperatures, although theadvantages of isentropic expansion of the cold feed gas would be thesame.

The pressure of the feed gas at point is preferably from about 2000 toabout 6000 p.s.. The feed gas pressure is based on several factorsincluding: (l) higher gas pressure reduces the equilibrium amount ofwater vapor retained in the gas, and the greater the amount of liquidwater removed in the gas, and the greater the amount of liquid waterremoved in separator 101, the lesser the amount of water vapor to beremoved in vessel 102 and hence the smaller and less expensive vessel102 becomes; (2) a gas at high pressure is denser and for this reasonits thermodynamic and physical properties are better suited to heattransfer. By taking advantage of this phenomenon and holding the gas athigh pressure until it is cooled in the process to the least possibletemperature before expansion, better heat transfer coefficients areobtained which permits the use of smaller and cheaper heat transferexchangers; (3) the feed gas pressure influences the operation of theexpansion engine 111. Thus, the greater the pressure expansion ratioacross expander 111 the greater the temperature drop and the greater thework available.

In general, the presently available single stage expanders are limitedby mechanical characteristics to a pressure ratio range of about 15/1maximum to about 5/1 minimum. Since the pressure after passage of thegas stream through expander 111 corresponds to the operating pressure ofcolumn 109, and since this column is operated at a pressure sufficientlyhigh to permit the use of methane evaporating at about atmosphericpressure as the refrigerant in the nitrogen condenser 172 (normallyabout 400 p.s..), it follows that an expansion ratio range of 5/1 to15/1 would tix the inlet gas pressures at 100 from about 2000 to about6000 p.s.. Should the gas be available at pressures greater than 6000p.s.., it would be used to advantage without any need for boostercompressors. For example, it would be possible to use dual stageexpanders in lieu of the single stage expander 111 to utilize inletpressures greater than 6000 p.s.. However, the design of such a dualstage compressor at the present time would be uneconomical. On the otherhand, if the gas pressure at point 100 is substantially less than 2000p.s.., it becomes desirable to compress the gas to at least about 2000p.s.. This is due to the fact that expander 111, when operated at aratio of less than 5/1, will not remove an optimum amount of work fromthe gas, nor will cooling of the gas before introduction into column 109be achieved to the desired extent. If the feed gas were isentropicallyexpanded at ambient temperature, the greatest possible work of expansionWould be obtained. However, it would be necessary to provide additionalcostly lower level refrigeration somewhere in the process to compensatefor not having done so by expansion machine. According to the presentinvention, while the Work of expansion obtained is somewhat less, thelowest possible level of refrigeration by expansion engine is achieved.

The pressure of the feed gas at point 100 limits the degree of expansionacross expander 111 since, according to this invention, the dischargepressure from the expansion machine 111 must be sufficiently high topermit methane refrigeration by boiling at about one atmosphere tocondense nitrogen in condenser 172. We have found substantially lowerpressures to be unsatisfactory. For example, it has been suggested thatmethane and nitrogen be separated in a column operated at p.s.. Thispressure indicates a saturation temperature for pure nitrogen of 275 F.Under this condition should it be desired to recover pure nitrogen it isnecessary to provide high pressure nitrogen refrigeration in order toachieve a condenser temperature of at least as low as 275 F. Accordingto the present invention the valuable nitrogen is readily recoveredwithout the need for nitrogen or other low level refrigeration. Therequired heat transfer at the top of the column 109 is achieved by theuse of methane. In the operation of the column 109 all refrigeration tokeep the column in heat balance and provide down tlowing reflux liquidsto effect gas separation is easily provided by a form of cascaderefrigeration system, the lowest level of which is methane at about oneatmosphere and at about -258.7 F. (saturated vapor). This is suicient tocondense the nitrogen in condenser 172 since at about 400 p.s.i.nitrogen boils at -240.4 F. If the column were operated at pressuressubstantially below 400 p.s.i., the boiling point of the nitrogen at thetop of the column would be lower than the boiling point of methane atone atmosphere, that is at -258.7 F. Thus, it would be necessary to usesome colder material to provide refrigeration on condenser 172. Theprovision of such a refrigerant would necessitate additional compressionlwork. This compression work is eliminated by the practice of thisinvention.

The nitrogen produced by our invention from the top of column 109 may beused in ammonia synthesis. Since the nitrogen produced by the presentinvention is already at a high pressure, less compression is required toraise the nitrogen to the pressure of ammonia synthesis. Thus, by usingnitrogen from the process of this invention, the eciency of ammoniaproduction is increased. The nitrogen from separation may also bereadily liquitied at pressure prior to flashing to a saleable liquidproduct and held in storage prior to delivery.

The limiting distillation tower pressure in column 109 would bedetermined both by the critical properties of any given feednitrogen-hydrocarbon composition, and the desired product purities.Accordingly, the present invention is not limited to the use of anyspecific pressure in column 109, so long as the nitrogen can becondensed by boiling methane. In the present invention, the removal ofwork in expansion of the feed gas is the maximum believed to beattainable, consistent with the minimizing of the amount ofrefrigeration work required to condense the nitrogen at the top ofcolumn 109. Under the conditions in expander 111, up to about 15 molepercent liquid is formed during the expansion. This percentage ofliquids formed is considered the practical limit for mechanical reasonswith the presently available expanders. However, the invention is notlimited to operating expander 111 in the liquid range. The expander 111could just as well be operated with discharged gas as saturated vapor oreven superheated vapor.

In addition to nitrogen, various other inerts occur in natural gasesincluding helium, and argon. Both of these gases are valuable in pureform. According to the present invention it is possible to obtain argonand helium in substantially pure form. For example, the argon because ofits relative volatility preferentially tends to separate out with thenitrogen product and to accumulate in the liquid nitrogen from thedistillation tower 109. Thus, as an alternated embodiment of theinvention, an argon rich side stream may be withdrawn at someintermediate point in the distillation tower 109, generally above thefeed point, said stream being treated by absorption, distillation orother appropriate means to concentrate and purify argon and return thenitrogen and methane to tower 109. The argon might also be recoveredfrom overhead nitrogen product from tower 109 by appropriate means suchas adsorption or distillation. It is also possible to recover argondirectly from the liquid nitrogen product in drum 144. Again, means suchas adsorption or distillation can be utilized.

Helium concentrations in natural gas range from O to as much as 8.5volume percent. Generally, a helium containing natural gas will containabout as much nitrogen as helium or more likely more nitrogen thanhelium.

A natural gas containing helium can be treated in the same manner as anyother natural -gas in tower 109. In the tower, by virtue of its highvolatility, the helium tends to collect overhead with the nitrogen. Toaccomplish separation of the helium from the nitrogen, a stagedcondenser or dephlegmator is added above condenser 172. The capacity ofthe cascade refrigeration system would have to be increased and anitrogen loop added to provide the necessary low temperaturerefrigeration required for condensing the nitrogen from the helium. Tohold helium losses to a minimum, condensation of the nitrogen shouldoccur at lower pressures. Therefore, the liquid nitrogen contatininghelium is throttled into the dephlegmator to lower pressure than intower 109. Part of the totally condensed nitrogen is used to refrigeratethe dephlegmator. The portion of liquid nitrogen from the tower in line140 so utilized is a function of the demand for liquid nitrogen productat line 145. The vapor helium from the dephlegmator would be furtherpuried by conventional means. After extraction the cold helium can beincorporated in the cascade refrigeration system in a separate pass totake advantage of its refrigeration value.

As has been indicated above, the incoming gas stream from point isnormally dried rst by passage through separator 101 and vessel 102. Thisis to prevent formation of ice in the gas as the temperature is loweredin the process. Similarly, the heavy hydrocarbons are removed frorn thesystem by separator 107 via separator 112. This prevents possibleplugging due to the accumulation of heavier hydrocarbons in the system.However, the ice and heavier hydrocarbons may be removed by othertechniques, for example, by the use of adsorbent materials or bychemical treatment. Similarly the removal of residual carbon dioxide andsulfur compounds may be accomplished by any other chemical or physicaltreatment. These procedures are Well-known to those skilled in the artand will not be further described here.

While the foregoing discussion has been with particular reference to theseparation of a gas stream containing about 60 volume percent nitrogen,the process is not limited thereto. Any percentage of nitrogen fromabout 0 mole percent to 100 mole percent can be separated by the processof this invention although obviously separation would not ordinarily benecessary where the feed gas is essentially pure.

As shown in the drawing, the cooling effect for heat exchanger issupplied by counterflowing cold methane rich product. However, thecooling effect for this heat exchanger need not be supplied in thisfashion. For example, some of the ethylene or propane refrigerantstreams may be utilized to provide the necessary cooling a this point.Accordingly, such variations are contemplated by the present invention.

The degree of separation achieved in tower 109 may be varied accordingto the desired purity of the nitrogen and methane products. This may beaccomplished by varying the number of separation stages in the column,as well as by altering the amount of heat re-y moved from the column viacondenser 172. Similarly, the ratio of gas to liquid in the nitrogenproducts removed frorn the top of column 109 may be varied according towhether the one type of product or the other type of product ispreferred.

The cascade system employed to provide additional refrigeration in thesystem is unique in incorporating cold vaporizing and gaseous productstreams, including waste products, together with cold low pressurerefrigerant streams as cooling, liquefying and subcooling media for theWarm compressed refrigerants. While the foregoing discussion of thecascade system has been with particular reference to propane, ethyleneand methane, other refrigerants such as ammonia, other hydrocarbons,halocarbons and the like could be used equally as well. The selection ofthe particular refrigerants in any situa- 13 tion is dictated byavailability, cost and efliciency. For any given system, those skilledin the art will be readily able to select the refrigerants to beemployed, based on the criteria set forth above.

The number of cold streams employed in the cascade refrigeration processis not fixed. For example, in methane chiller 122 five cold streams areshown. However, the number and composition of the cold streams utilizedmay be varied. For example, they could be interchanged for other streamselsewhere in the total system or interchanged for different productsplits than are specifically illustrated in the drawing. Accordingly,such variations n the cascade system are included within the scope ofthe present invention.

As shown in the drawing, the methane refrigerant is pumped by compressor161 through nitrogen condenser 172, methane subcooler 120, line 177,methane chiller 122, line 178, methane cooler 158 and finally via line179 to the compressor 161. As an alternative, methane refrigerant can bepartially cooled by passing it through a second reboiler parallel to,and mechanically dependent of, the natural gas reboiler 108 in the baseof the tower 109. In such case, cold streams above-described asimparting refrigeration to methane, can be shifted to natural gas feedand/or propane and ethylene refrigerant streams. Obviously, whatever thearrangement of the heat exchanger system, it must be in heat balance. Asecond alternative would be to effect all tower reboiling by methanerefrigerant and all natural gas precooling against product streams.

In yet another embodiment of the invention hydrogen from natural gas orfrom a synthetic gas may be recovered. It is also possible toincoroprate liquid nitrogen scrubbing of impure hydrogen in conjunctionwith the gas separation process. In such a case additional refrigerationwould be provided by the cascade system to form more liquid nitrogenfrom tower 109 at line 140. This extra liquid nitrogen, subcooled, andthen downowed in a separate tower scrubs upowing cold impure hydrogenfree of all higher boiling materials except nitrogen and a trace ofcarbon oxides. The hydrogennitrogen vapor from the top of such ascrubbing tower is used in a separate pass in the cascade refrigerationsystem for its refrigeration value and then utilized elsewhere, as forexample, in the synthesis of ammonia. the liquid, nitrogen rich, bottomsfrom the scrubbing tower can be further treated to remove undesirablecomponents such as carbon oxides and the remaining material recycled totower 109 as an intermediate feed or reux at some point between feedline 116 and the liquid nitrogen overhead, line 140. In this way liquidnitrogen and minor components such as hydrogen and methane from thebottom of the hydrogen scrubber are continuously recycled and reclaimed.In addition, the presence of an intermediate feed in distillation tower109 relieves the separation by acting as additional reflux to decreasethe number of separation stages required and/or to provide increasedseparation of products.

Many other modifications of the invention will be apparent to thoseskilled in the art from the foregoing disclosure. Accordingly, theinvention is not -to be limited to the details described, but rather isof the full scope of the appended claims.

We claim:

1. A process for the reduction of the amount of inerts in a mixturecomprising a combustible hydrocarbon including methane and inert gasesincluding nitrogen, which comprises:

(a) cooling a stream of high pressure gas containing combustiblehydrocarbons and inert gases including nitrogen;

(b) expanding the cooled stream under substantially isentropicconditions to cool the gas and remove work therefrom;

(c) passing the expanded stream into a separation zone operated atsufliciently high pressure to permit the use of a methane refrigerant atits boiling point to remove heat from the top of said zone;

(d) forming said methane refrigerant at its boiling point by the stepscomprising:

cooling and condensing a propane refrigerant with water, condensing andsubcooling said propane with cold product streams and recycled lowpressure propane refrigerant vapor;

cooling and condensing an ethylene refrigerant with water, cold productstreams, and vaporizing propane;

subcooling said ethylene with cold product streams and recycled lowpressure ethylene refrigerant vapor;

cooling said methane refrigerant with water, cold product streams, andrecycled low pressure methane refrigerant Vapor;

condensing methane with evaporating ethylene and evaporating hydrocarbonproduct; and

subcooling methane with cold product streams and recycled low pressuremethane refrigerant vapor;

(e) passing said methane to the top of said zone; and

(f) removing substantially pure nitrogen product stream from the top ofsaid zone and substantially pure methane product stream from the bottomof saidzone.

2. A process for the reduction of the amount of inerts in a mixturecomprising a combustible hydrocarbon and inert gases including nitrogen,which comprises:

(a) cooling a stream of high pressure gas containing combustiblehydrocarbons and inert gases including nitrogen;

(b) expanding the cooled stream under substantially isentropicconditions to cool the gas and remove work therefrom;

(c) passing the expanded stream into a separation zone operated atsufficiently high pressure to permit the use of methane at its boilingpoint to remove heat from the top of said zone;

(d) forming said methane at its boiling point by the steps comprising:

condensing propane with water, condensing and subcooling propane withcold product streams and recycled low pressure propane refrigerantvapor;

condensing ethylene with water, cold product streams, and vaporizingpropane;

subcooling ethylene with cold product streams, and recycled low pressureethylene refrigerant vapor;

cooling methane with water, cold product streams, and recycled lowpressure methane refrigerant vapor;

condensing methane with evaporating ethylene;

and

subcooling methane with cold product streams and recycled low pressuremethane refrigerant vapor;

(e) passing said methane to the top of said zone; and

(f) removing substantially pure nitrogen from the top of said zone andsubstantially pure methane from the bottom of said zone.

3. A process for the reduction of the amount of inerts in a mixturecomprising a combustible hydrocarbon and inert gases including nitrogen,which comprises:

(a) cooling a stream of high pressure gas containing combustiblehydrocarbons and inert gases including nitrogen;

(b) expanding the cooled stream under substantially isentropicconditions to cool the gas and remove work therefrom;

(c) passing the expanded stream into a separation zone operated atsufficiently high pressure' to permit the use of methane at its boilingpoint to remove heat from the top of said zone;

(d) forming said methane at its boiling point at a pressure of at leastone atmosphere in a separate pass by the steps comprising:

condensing propane with water, condensing and subcooling propane withcold product streams and recycled 10W pressure propane refrigerantvapor;

condensing ethylene with water, cold product streams, and vaporizingpropane;

subcooling ethylene with cold product streams and recycled low pressureethylene refrigerant vapor;

cooling methane with Water, cold product streams, and recycled lowpressure methane refrigerant vapor;

condensing methane with evaporating ethylene; and

subcooling methane with cold product streams and recycled low pressuremethane refrigerant vapor;

(e) passing said methane to the top of said zone; and

(f) removing substantially pure nitrogen from the top of said zone andsubstantially pure methane from the bottom of said zone.

4. A process for the reduction of the amount of inserts in natural gascomprising a combustible hydrocarbon and inert gases including nitrogen,which comprises:

(a) cooling a stream of high pressure gas containing combustiblehydrocarbons and inert gases including nitrogen;

(b) expanding the cooled stream under substantially isentropicconditions to cool the gas and remove Work therefrom;

(c) passing the expanded stream into a separation zone operated atsufliciently high pressure to permit the use of methane at its boilingpoint to remove heat from the top of said zone;

(d) forming said methane at its boiling point by the steps comprising:

condensing propane with water, condensing and subcooling propane withcold product streams and recycled low pressure propane refrigerantvapor;

`condensing ethylene With water, cold product streams, and vaporizingpropane;

subcooling ethylene -with cold product streams and recycled low pressureethylene refrigerant vapor;

cooling methane with water, cold product streams, and recycled lowpressure methane refrigerant vapor;

condensing methane with evaporating ethylene; and subcooling methanewith cold product streams and recycled low pressure methane refrigerantvapor;

(e) passing said methane to the top of said zone; and

(f) removing substantially pure nitrogen from the top of said zone andsubstantially pure methane from the bottom of said zone.

5. A process for the reduction of the amount of inerts in a mixturecomprising a combustible hydrocarbon and inert gases including nitrogen,which comprises:

(a) cooling a stream of high pressure gas at about 2000 to 6000 p.s.i.a.containing combustible hydrocarbons and inert gases including nitrogen;

(b) expanding the cooled stream under substantially isentropicconditions to cool the gas and remove Work therefrom;

(c) passing the expanded stream into a separation zone operated atsufficiently high pressure to permit the use of methane at its boilingpoint to remove heat from the top of said zone;

(d) forming said methane at its boiling point by the steps comprising:

condensing propane with water, condensing and subcooling propane withcold product streams and recycled low pressure propane refrigerantvapor;

condensing ethylene with water, cold product streams and vaporizingpropane;

subcooling ethylene with cold product streams and recycled low pressureethylene refrigerant vapor;

cooling methane with water, cold product streams, and recycled lowpressure methane refrigerant vapor;

condensing methane With evaporating ethylene;

and

subcooling methane with cold product streams and recycled low pressuremethane refrigerant vapor;

(e) passing said methane to the top of said zone; and

(f) removing substantially pure nitrogen from the top of said zone andsubstantially pure methane from the bottom of said zone.

6. A process for the reduction of the amount of inerts in a mixturecomprising a combustible hydrocarbon and inert gases including nitrogen,which comprises:

(a) cooling a stream of high pressure gas at about 2000 to 6000 p.s.i.a.containing combustible hydrocarbons and inert gases including nitrogen;

(b) expanding the cooled stream under substantially isentropicconditions to about 400 p.s.i.a. to cool the gas and remove worktherefrom;

(c) passing the expanded stream into a separation zone operated atsuiciently high pressure to permit the use of methane at its boilingpoint to remove heat from the top of said zone;

(d) forming said methane at its boiling point by the steps comprising:

condensing propane with water, condensing and subcooling propane withcold product streams and recycled low pressure propane refrigerantvapor;

condensing ethylene with Water, cold product streams, and vaporizingpropane;

subcooling ethylene with cold product streams and recycled low pressureethylene refrigerant vapor;

cooling methane -With Water, cold product streams, and recycled lowpressure methane refrigerant vapor;

condensing methane with evaporating ethylene; and subcooling methanewith cold product streams and recycled low pressure methane refrigerantvapor;

(e) passing said methane to the top of said zone; and

(f) removing substantially pure nitrogen from the top of said zone andsubstantially pure methane from the bottom of said zone.

7. A process for the reduction of the amount of inerts in a mixturecomprising a combustible hydrocarbon and inert gases including nitrogen,which comprises:

(a) cooling a stream of high pressure gas containing combustiblehydrocarbons and inert gases including nitrogen;

(b) expanding the cooled stream under substantially isentropicconditions to cool the gas and remove work therefrom;

(c) passing the expanded stream into a separation zone operated atsuiciently high pressure to permit the use of methane at its boilingpoint to remove heat from the top of said zone;

(d) forming said methane at its boiling point by the steps comprising:

Acondensing propane with Water, condensing and subcooling propane withcold product streams and recycled low pressure propane refrigerantvapor;

condensing ethylene with Water, cold product streams, and vaporizingpropane;

subcooling ethylene with cold product streams and recycled low pressureethylene refrigerant vapor;

cooling methane with water, cold product streams,

17 and recycled low pressure methane refrigerant vapor; condensingmethane with evaporating ethylene;

and

subcooling methane with cold product streams and recycled low pressuremethane refrigerant vapor;

(e) passing said methane to the top of said zone;

(f) removing substantially pure nitrogen from the top of said zone andsubstantially pure methane from the bottom of said zone; and

(g) converting said nitrogen to ammonia.

8. A process for the reduction of the amount of inerts in a mixturecomprising a combustible hydrocarbon and inert gases including nitrogen,which comprises:

(a) cooling a stream of high pressure gas containing combustiblehydrocarbons and inert gases including nitrogen;

(b) expanding the cooled stream under substantially isentropicconditions to cool the gas and remove work therefrom;

(c) passing the expanded stream into a separation zone operated atsufficiently high pressure to permit the use of methane at its boilingpoint to remove heat from the top of said zone;

(d) forming said methane at its boiling point by the steps comprising:

condensing propane with water, condensing and subcooling propane withcold product streams and recycled low pressure propane refrigerantvapor;

condensing ethylene with water, cold product streams, and vaporizingpropane;

subcooling ethylene with cold product streams and recycled low pressureethylene refrigerant vapor;

cooling methane with Water, cold product streams, and recycled lowpressure methane refrigerant Vapor;

condensing methane With evaporating ethylene;

and

subcooling methane with cold product streams and recycled low pressuremethane refrigerant vapor;

(e) passing said methane to the top of said zone;

(f) removing substantially pure nitrogen from the top of said zone andsubstantially pure methane from the bottom of said zone; and

(g) separating argon from said nitrogen.

9. A process for the reduction of the amount of inerts in a mixturecomprising about 40 mole percent methane and about 60 mole percentnitrogen, which comprises:

(a) cooling a stream of high pressure gas at about 2000 to 6000 p.s.i.a.containing combustible hydrocarbons and inert gases including nitrogen;

(b) expanding the cooled stream under substantially isentropicconditions to about 400 p.s.i.a. to cool the gas to about 230 F. andremove work therefrom;

(c) passing the expanded stream into a separation zone operated atsufficiently high pressure to permit the use of methane at its boilingpoint to remove heat from the top of said zone;

(d) forming said methane at its boiling point by the steps comprising:

condensing propane with water, condensing and subcooling propane withcold product streams and recycled low pressure propane refrigerantvapor;

condensing ethylene with water, cold product streams, and vaporizingpropane;

subcooling ethylene with cold product streams and recycled low pressureethylene refrigerant vapor;

cooling methane with water, cold product streams, and recycled lowpressure methane refrigerant vapor;

condensing methane with evaporating ethylene;

and

subcooling methane with cold product streams and recycled low pressuremethane refrigerant vapor,

(e) passing said methane to the top of said zone; and

(f) removing substantially pure nitrogen from the top of said zone andsubstantially pure methane from the bottom of said zone.

10. A process for the reduction of the amount of inerts in a mixturecomprising a combustible hydrocarbon and inert gases including nitrogen,which comprises:

- (a) cooling a stream of high pressure gas at about 2000 to 6000p.s.i.a. containing combustible hydrocarbons and inert gases includingnitrogen;

(b) expanding the cooled stream under substantially isentropicconditions to about 400 p.s.i.a. to cool the gas and remove worktherefrom;

(c) passing the expanded stream into a separation zone operated atsufficiently high pressure to permit the use of methane at its boilingpoint to remove heat from the top of said zone;

(d) forming said methane at its boiling point by the steps comprising:

condensing propane with water, condensing and subcooling propane lwithcold product streams and recycled low pressure propane refrigerantvapor;

condensing ethylene with Water, cold product streams, and vaporizingpropane;

subcooling ethylene with cold product streams and recycled low pressureethylene refrigerant Vapor;

cooling methane with Water, cold product streams, and recycled lowpressure methane refrigerant vapor;

condensing methane with evaporating ethylene; and

subcooling methane with cold product streams and recycled loW pressuremethane refrigerant vapor;

(e) passing said methane to the top of said zone; and

(f) removing substantially pure nitrogen from the top of said zone andsubstantially pure methane from the bottom of said zone.

References Cited UNITED STATES PATENTS 2,355,589 8/1944 Brandt 62-31 XR2,556,850 6/1951 Ogorzaly 62-40 XR 2,557,171 6/1951 Bodle 62-31 XR2,716,332 8/1955 Haynes 62-32 XR 2,823,528 2/1958 Ea'kin 62--262,960,837 11/ 1960 Swenson 62-40 XR 2,990,690 7/ 1961 Martin 62-27 XR3,020,723 2/ 1962 De Lury 62-40 XR 3,315,477 4/1967 Carr 62-40 XR NORMANYUDKOFF, Primary Examiner A. F. PURCELL, Assistant ExaminerA U.S. Cl.XR. 62-40, 22, 38

